Fructose can exist in an anhydrous crystalline form as orthorhombic, bisphenoidal prisms which decompose at about 103.degree.-105.degree. C. Hemihydrate and dihydrate crystalline forms are also known, but it is preferable to void the formation of these species inasmuch as they are substantially more hygroscopic than the anhydrous form and have melting points close to room temperature which makes these crystalline forms of fructose very difficult to handle. Accordingly, as used herein, "crystalline fructose" shall refer to anhydrous crystalline fructose unless expressly stated otherwise in context.
Crystalline fructose is generally prepared by one of three ways, i.e., crystallization from solvent, crystallization from an aqueous solution, and drying of a fructose syrup.
It is possible to simply produce a dried fructose sweetener (DFS). In a DFS process, a high fructose stream derived from fractionation is dried in a rotary dryer, then sized in a classifier containing screens and grinders. U.S. Pat. No. 4,517,021 describes the preparation of such a granular, semi-crystalline, solid fructose which comprises less than about 2% water by weight. The patent discloses that about 60 weight percent of the product is crystalline fructose, and less than 35 weight percent is amorphous fructose. A drum dryer is used, with air having an initial temperature of 50.degree.-80.degree. C. A portion of the solid fructose product may be recycled as the crystallization initiator.
One disadvantage of a DFS process is that the product cannot be called pure fructose because it is a total sugar product and does not meet the Food Chemicals Codex criteria for "fructose." Moreover, since it is not completely crystalline, it is more hygroscopic and thus harder to handle in humid conditions than crystalline fructose.
Crystalline fructose can be prepared by a process wherein an organic solvent, such as denatured ethyl alcohol, is mixed with a high-fructose stream (95% d.s.b.). This stream crystallizes as it is cooled to form pure crystalline fructose. The product is centrifuged to separate it from the mother liquor, desolventized, and dried. U.S. Pat. No. 4,199,374 describes a process for producing crystalline fructose by the use of organic solvent. Fructose is crystallized from a high fructose corn syrup mixed with ethanol. The solution is seeded with fine crystals, e.g., particle size of 0.05 mm to 0.50 mm, of fructose or glucose and allowed to cool. The crystals are harvested by filtration, centrifugation or other suitable means. These crystals are then washed with alcohol and dried under vacuum. The moisture content of the alcohol and syrup must be carefully controlled in this process in order to obtain free-flowing fine crystals of fructose.
An aqueous process can be used to produce crystalline fructose. An aqueous crystalline fructose process typically starts with a high fructose feed stream at an elevated temperature which is cooled to crystallize the fructose from solution. A number of references describe such a process.
In U.S. Pat. No. 3,513,023, crystalline, anhydrous fructose is obtained from an aqueous solution of fructose (min. 95% d.s.). The pH of the solution must be between 3.5 and 8.0. The fructose solution is concentrated under vacuum until the water content is between 2 and 5%. The solution is cooled to 60.degree.-85.degree. C., seeded with crystalline fructose, and stirred vigorously while the temperature is maintained at 60.degree.-85.degree. C. The patentee states that a crystalline mass results which, after slow cooling, can be crumbled or ground and subsequently dried to produce a non-sticking, free-flowing, finely-crystalline powder. The process is said to avoid the formation of the glass phase product which ordinarily results when fructose solutions of this type are concentrated in a vacuum and allowed to cool in the usual manner.
In U.S. Pat. No. 3,883,365, fructose is crystallized from an aqueous fructose/glucose solution of 90% d.s. and containing 90-99% (d.s.b.) fructose. The solution is saturated (58.degree.-65.degree. C.). The fructose is crystallized from the solution by adding fructose crystals of homogeneous size, e.g., crystals of 5 to 10 micrometers as suspended in isopropanol or larger crystals, e.g., 80 to 100 micrometers, as dry. The formation of new crystals is minimized by keeping the distances of the seed crystals from each other suitably short and maintaining the degree of supersaturation between 1.1 and 1.2. The volume of the solution is increased, either continuously or stepwise, as the crystallization proceeds. The optimum pH of the fructose solution is said to be 5.0. The crystals so obtained reportedly have an average crystal size between 200 and 600 micrometers. Centrifugation is used to separate the crystals from the solution.
U.S. Pat. No. 3,928,062 discloses that anhydrous fructose crystals are obtained by seeding a solution containing 83-95.5% (dry basis) total sugar comprising 88-99% fructose. Crystallization may be accomplished by simply cooling the solution under atmospheric pressure or by evaporating water under reduced pressure. Formation of the hemihydrate and dihydrate are avoided by carrying out the crystallization within a certain range of fructose concentrations and temperatures. This range lies within the supersaturation area below the point at which the hemihydrate begins to crystallize out. The solution is seeded with 1 to 4% by weight of seed crystals (based on the weight of the solution) preferably having a particle size of 0.06 mm to 0.1 mm. The addition of seed crystals may be achieved using a form of massecuite which was previously prepared by suspending the crystals in the fructose solution.
In U.S. Pat. No. 4,199,373, crystalline fructose is produced by seeding a fructose syrup (88-96% d.s.b.) with 2-15 weight percent fructose seed crystals having a particle size of less than 250 micrometers, e.g., 50 to 150 micrometers, and permitting the seeded syrup to stand at about 50.degree. to 90.degree. F. at a relative humidity below 70%. Crystallization is said to require 2 to 72 hours. The crystalline product produced by the process is in the form of large pellets.
U.S. Pat. No. 4,164,429 describes a process and apparatus for producing crystallization seeds. A series of centrifugal separations are employed to select seed crystals from the seeded solution which fall within a predetermined size range.
U.S. Pat. No. 4,666,527 describes a process for continuously crystallizing fructose using a seed crystallization tank and a separate crystallization tank. A temperature gradient is maintained between the upper and lower portions of each tank and crystals progress from each of said upper portions to said lower portions and thereby grow. Initially, powdered seed crystals are added to the seed crystallization tank in an amount of 1 to 5%, but overflow from the crystallization tank is used as seed once a steady state of continuous operation has been established.
The selection of crystallizers and the importance of seeding in batch crystallization in general is discussed by W. J. Genck, "Selection of Crystallizers," Chemical Processing, December, 1988, pp. 62-67. It is noted that beginning conditions in batch crystallization are important because the initial shower of nuclei or deliberate seeding becomes part of the crystal size distribution and this initial activity can, in fact, control the final product.
The production of refined cane sugar (sucrose) is described by H. M. Pancoast et al., Handbook of Sugars, pp. 5-7. After centrifugation, washing and drying, the crystals of refined cane sugar can be classified according to size with screen classifiers, but Pancoast et al. note that most of the sugar is not classified because experience crystallizer operators can hold the crystal size distribution relatively constant.
The crystallization of fructose from solution produces a slurry of crystalline fructose product in mother liquor, i.e., a massecuite. The crystalline fructose product is isolated from the mother liquor, washed and dried. The dried crystalline product is then typically manipulated to ensure that it will have a desirable particle size distribution. Such manipulation generally entails dividing the particles of crystalline fructose into portions on the basis of particle size (i.e., particle size classification) and typically employs a plurality of screens. For example, the crystalline fructose product will typically be classified by screening it sequentially through two screens (the second screen having smaller openings) to retain oversized crystals and/or crystal agglomerates on the first screen and allow crystal fines to pass through the second screen. The crystalline fructose product is retained on the second screen and conveyed for further product handling.
Because products having different mean particle sizes and/or particle size distributions may be desired, the typical classification described above can be modified to introduce one or more intermediate screens upon each of which a product having a certain particle size different for each screen is retained.